1. Field of the Invention
The present invention generally relates to an optical system of telescope, and more particularly to an optical system having high field brightness.
2. Description of the Prior Art
FIG. 1 of the attached drawings shows a conventional optical system 9 of telescope. The conventional optical system 9 comprises an objective lens 90 that collects incident light and transmits the incident light along a light path, a prism 91 disposed on the light path for refracting and converging the light beam in order to reduce the whole optical length of the optical system 9, an imaging lens group 92 disposed on the light path for adjusting the focus and determining an image plane 94 of the optical system 9, and two pieces of flat glass 93 disposed in front of the image plane 94. The imaging lens group 92 comprises four lenses, which are respectively first, second, third, and fourth lenses 95-98. The first lens 95 is a positive lens, the second lens 96 is a negative lens, the third lens 97 is a positive lens, and the fourth lens 98 is a positive lens. However, the conventional optical system 9 has a problem on field brightness, which can be clearly observed from the test chart shown in FIG. 2. The brightness at a circumferential edge is much lower than that at the center area. The plot shown in FIG. 2 indicates that the relative brightness is only 36% at the radius of 3 mm. The edge brightness is very low, which makes the edge of the field of view obscure or even dark.
Additionally, the first through fourth lenses 95-98 are all made of glass whose refractive indices are all higher than 1.80, as shown in the following table:
Refractive IndexAbbe coefficientLensType of Glass(Nd)(Vd)First lens 95S-LAH531.806140.90Second lens 96S-NPH11.808122.80Third lens 97S-LAH651.804046.60Fourth lens 98S-LAH651.804046.60
For a glass lens, generally, the higher the refractive index is, the lower the rigidity is. Therefore, the lenses 95-98 having high refractive indexes are difficult to process. Furthermore, the cost of the conventional optical system 9 is high for using such glass lenses having high refractive indexes.
Hence, an improved optical system is desired to overcome the above-mentioned disadvantages of the prior art.